Dmitriy A. Plugin, PhD, Docent, Arkadiy N. Plugin, DSc, Professor,

Aleksei A. Plugin, PhD, Olga S. Borzyak, PhD

Ukrainian State Academy of Railway Transport, Kharkov, Ukraine

ELECTRO-CORROSION OF CONSTRUCTIONS OF RAILWAY TUNNELS

The operational reliability of concrete, reinforced concrete and stone structures and rail supports depends on ability of their constructions to resist the destruction of leakage current and stray current. The existent notions and current standards take into account that destroying influence renders only a direct current and only in the anodal zones of metallic constructions and armature of reinforced concrete constructions. However recently the researchers of Ukrainian State Academy of Railway Transport got a new information and developed the hypotheses that a cement stone and concrete is also exposed to electro-corrosion from the action of direct current. The fact that in some tunnels, which are built and exploited in accordance to current standards, unforeseen considerable corrosive damages are marked.

Character of electric current flowing through concrete and cement stone of constructions. A railway contact line energizes from traction substation and divided on sections (fig. 1). Sections are isolated one from other by sectional delimiters and insulating rail joints.

Fig. 1 – Scheme of the longitudinal sectionalizing of contact line [1]: A, B are sectional delimiters

The rails are the return line of contact line of every section. At a motion of train on a section there is an area with positive potential on rails under an electric locomotive. Thgis area moves at a speed of the train (fig. 2). When the train leaves a section the potential disappears, so the a current and potential are not direct, but pulsating unidirectional. Some part of pulsating unidirectional current flows down from rails through rail-fastening clip, sleepers (including through the concrete of sleeper) and ballast in ground (fig. 2).

Fig. 2 – The scheme of traction current flowing down from a rail track and distributing of potentials along rails at the one-sided power of section [2]: TS – traction substation; EL – electric locomotive; І – traction current; Іr – current in rails; Іg – current in ground; Ur-g – potential difference “rail – ground”; 1 – values at the complete isolation of rails from ground; 2 – actual values

A current from rails flows also through constructions of tunnels in a ground.

Measured potential difference between a rail and a distant point of earth, between the tunnels constructions and distant point of earth (fig. 3 – 5) testify to the presence of such currents [3 – 7].

Fig. 3 – The scheme of potential measurement on the rail of the electrified track, ballast and surface of facing the tunnel: 1 – rail, 2 - ballast, 3 - facing the tunnel, 4 - measuring equipment, 5 - grounding electrode

a)

b)

Fig. 4 – Dependence of potential in relation to the distant point of earth U, V, from time Hour:Min:Sec: a – on the rail, Umax = 115,6 В (12:06:05); b – on the concrete surface of facing the tunnel, Umax = 2,99 В (12:06:36) [3]

a)

b)

Fig. 5 – Dependence of potential in relation to the distant point of earth U, V, from time Hour:Min:Sec: a – on the rail, Umax = 49,95 В (12:52:25); b – in a ballast under the railway sleeper, Umax = 3,657 В (12:53:51) [3]

Basic hypothesis of researches. Pulsating unidirectional electric potential and current stipulates the electro-corrosion of concrete. This electro-corrosion consists in intensification of leaching of cement stone and formation of cracks in a concrete due to the dissolution and electro-migration carry-out of calcium hydroxide in a water or water-saturated ground [6 – 10]. The amount of the calcium hydroxide carried out from the concrete depends on the amount of the transferred electric charge [8, 9].

Method of experimental researches of the pulsating unidirectional electric potential and current influence on a concrete. Experimental researches were done to prove the hypothesis of the destroying action of pulsating unidirectional potential and current on a concrete. The special laboratory installation for such action concrete was developed and made [6 – 9]. Samples-cubes of sizes 100×100×100 mm made from a concrete by strength about 10 MPa were made for researches. Composition of concrete on 1 m3: cement – 167 kg, broken stone – 1310 kg, sand – 667 kg, water – 200 liters, W/C = 1,2. Steel perforated plates - electrodes were set on the top and bottom face of samples.

Samples-cubes were placed in the capacity of the laboratory installation in a water on ¾ of their height. On the top and bottom plates - electrodes with help of the programmable power source the feed of potential difference was realized by the regime 7 minutes – turned on, 10 minutes – turned off. Such regime approximately corresponds to the intervals of trains passing. The influence of voltage of 40, 15 and 5 V at a stream of water through the capacity was researched. Such voltages correspond to the potential sizes on rails. Control samples were in a running and stagnant water without influence of current.

The readings of voltage, strengths of current, electric resistance of sample registered by the measuring devices and automatically recorded on a personal computer. After the protracted influence of pulsating unidirectional potential and current and running water, and also exposure of running and stagnant water on the control samples the compression strength, loss of strength, non-pressure water-permeability were determined. Physical-chemical researches – X-ray phase analysis, infrared spectroscopy, light and scanning electronic microscopy also were done.

The results of research of influence of pulsating unidirectional electric potential and current on a concrete. Graphs of dependence of strength of current I in samples from time at the protracted influence of pulsating unidirectional electric field at 40, 15 and 5 V are presented on a fig. 7–a. The current at the end of every impulse is less, than at the beginning of impulse. Dependence of this difference of current at the beginning and at the end of every cycle DI presented on a fig. 7–b. The carry-out of cations of Ca2+ from the concrete sample through its bottom face causes this difference of current. Dissolution of Ca(OH)2, electric potential and stream of water maintain this permanent carry-out.

a)                                                                   b)

 

Fig. 7 – Dependence of strength of current I, flowing through the concrete sample (a), and difference between the strength of current in a sample at the beginning and at the end of a cycle DI (b) from time t at the continuous action of pulsating unidirectional electric field at voltage of 40, 15, 5 V on the samples

Quantity of charge Q, which is carried-out from the investigated sample:

,                                        (1)

where: ΔIi is a difference between strength of current in a sample at the beginning and at the end of a cycle, A; ti  - duration of cycle, s; n - quantity of cycles.

Mass m Ca(OH)2, which is carried-out together with cations of Сa2+ from the investigated sample, determined by the size of Q:

,                                           (2)

where: M – molecular mass of Сa(OH)2, 74 g/mole; F – a number of Faraday, 9,65´104 C/mole.

The sizes of DIi and ti for every cycle were put in (1) and (2) and got the data for the graphs of dependences of carried-out the quantity of charge Q, С and Сa(OH)2 m (% from its initial quantity) from time of electric field  action at voltages 40, 15 and 5 V (fig. 8).

a)                                                           b)

Fig. 8 – Dependence of electric charge quantity taken away from the concrete sample Q (a) and quantity of Ca(OH)2 (from initial) taken away from the sample, % (b), from time t at a voltage of 40 V (upper curves), 15 V (middle curves) and 5 V (lower curves)

On the graph (fig. 8) the size of taken away charge after 90 days was Q = 8500 C. It corresponds to the mass of taken away Ca(OH)2  m = 6,5 g, or 52 % from its initial quantity (fig. 5). As a result of calcium hydroxide carry-out the strength of concrete decreases, and its permeability increases and also its protective properties in relation to an armature are lost.

The speed of Ca(OH)2 carry-out from the sample at the voltage of 15 V less approximately in 2,5 times, and at 5 V – in 7 times. However, such speed of carry-out considerably will reduce the term of service of concrete constructions. By the graph on a fig. 8-b at the 5 V the Ca(OH)2 carry-out for 90 days made 6,4 % from an initial quantity. The complete Ca(OH)2 carry-out  at the 5 V will happen approximately after 1300 days, all of CaO of a cement – after 2600 days, and all of crystalline hydrates – after 5200 days or 14 years. The state of constructions at a similar conditions for the same time confirms the reality of such speed of concrete electro-corrosion.

It is set that the action of electric field at the 40 V stipulated the considerable increase of porosity and non-pressure water-permeability, and also the loss of mass. The loss of mass of samples practically coincided with calculation values. The strength of concrete in the zone of water flowing after 104 days of electric field action has decreased compared to a sample that was only in running water by 16 %, in stagnant water by 18 %. The results of experiments convincingly testify to considerable intensification of leaching under the action of electric field even at the 5 V.

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